In a transfer device provided in a multistage forging press machine in which a plurality of forging portions having dies and punches are provided side by side in a horizontal direction, a workpiece whose outer circumference has a circular shape is held by use of holding claws in each forging portion, and conveyed to the next forging portion.
Various shapes are used as the shapes of the holding claws for holding the workpiece whose outer circumference has a circular shape.
FIG. 6A shows an example in which holding faces of a pair of holding claws are formed into arc shapes. According to the example, an assured holding function can be expected when the outer shape of a workpiece coincides with the arc shape formed in each holding face. However, when the outer diameter of the workpiece is changed, the holding claws have to be replaced. In this case, facilities have to be suspended due to replacement of the holding claws and adjustment following the replacement, causing reduction in facility operation rate.
FIG. 6B shows an example in which two holding claws are provided in a front end of each of arms disposed vertically, so that a workpiece can be held at four points. Of the holding claws supporting the workpiece at the four points, the lower ones are fixedly positioned while only the upper ones can move. When the outer diameter of the workpiece is changed, the central position of the workpiece is changed. It is therefore necessary to replace the holding claws or adjust the positions of the holding claws, causing the same problem as in the case of FIG. 6A.
FIG. 6C shows an example in which, of holding surfaces of a pair of holding claws disposed to be opposed to each other on both sides of a workpiece in a horizontal direction, one is formed into a flat face and the other is formed into a V-shaped groove so that the workpiece can be held at three points. According to the example in which the holding claws support the workpiece at the three points, there also arises a problem that the central position of the workpiece is displaced (hereinafter also referred to as “misaligned”) when the diameter of the workpiece to be held is changed. This problem will be described with reference to FIG. 7A to FIG. 7C.
Assume that a large-diameter workpiece W1 shown in FIG. 7A is held by a holding claw 110 whose holding face is formed into a flat face and a holding claw 112 whose holding face is formed into a V-shaped groove, and the workpiece W1 has a workpiece center O1 in this case.
After that, the holding claws 110 and 112 are moved in closing directions respectively by one and the same amount, so as to hold a small-diameter workpiece W2 shown in FIG. 7B. Assume that the workpiece W2 has a workpiece center O2 in that case. As shown in FIG. 7B, a positional displacement (misalignment) δ appears between the workpiece centers O1 and O2.
The misalignment δ with which a workpiece is held causes deterioration in conveyance position accuracy for the workpiece. Thus, processing accuracy of a product to be manufactured is deteriorated.
Incidentally, shapes of holding claws for holding a circular workpiece have been, for example, disclosed in the following Patent Document 1 and Patent Document 2.
The following Patent Document 1 discloses an invention as to a “robot hand”, in which a front end part of each holding claw for holding a workpiece is made expandable and contractible in a longitudinal direction of the holding claw so that the front end part of the holding claw can be expanded and contracted in accordance with a change in outer diameter of the workpiece when the outer diameter of the workpiece is changed.
However, according to Patent Document 1, in which the holding claws support the workpiece at four points, there is no suggestion about the problem belonging to the aforementioned example in which holding claws support a workpiece at three points. In addition, an opening/closing mechanism of the holding claws is different from that in the present invention.
The following Patent Document 2 discloses an invention as to a “workpiece conveyance device”, in which a workpiece can be supported at three points by a flat face and a V-shaped groove.
In Patent Document 2, however, there is no suggestion about the problem belonging to the aforementioned example in which a workpiece is supported at three points, that is, the problem of misalignment occurring when the outer diameter of the workpiece is changed. In addition, there is no specific suggestion about a mechanism for opening/closing holding claws.
In a transfer device provided in a multistage forging press machine in which a plurality of forging portions having dies and punches are provided side by side in a horizontal direction, a so-called chucking error may occur when a workpiece is held by a pair of holding claws. The chucking error includes, for example, a failure that the workpiece is dropped or a failure that the workpiece is held in a state where the workpiece is inclined more obliquely than in a regular state.
Such a chucking error makes it difficult to form a product into a predetermined shape. In addition, the chucking error may interfere with the device to cause a device trouble. It is therefore necessary to detect the chucking error as soon as the chucking error occurs.
As a specific method for detecting an error in chucking a workpiece, the following Patent Document 3 discloses an example in which a voltage is applied to a pair of holding claws so that a chucking error can be electrically detected based on a voltage change appearing in a detection circuit when the workpiece is not located between the holding claws (that is, when there is a chucking error).
In the example disclosed in Patent Document 3, it is possible to detect the presence/absence of a workpiece, but there is a problem that it is impossible to detect a chucking error in a state where the workpiece inclined obliquely is held.
On the other hand, the following Patent Document 4 discloses an example in which a detection plate is attached to an intermediate portion of an operating rod moving in a vertical direction when three holding claws (fingers) holding a workpiece are operated to be opened/closed, and a moving amount of the detection plate is detected by an eddy-current type sensor, so that waveform data obtained by the sensor can be regarded as equivalent to an opening amount of the holding claws, and abnormality can be detected based on the waveform data.
However, according to the example disclosed in Patent Document 4, the sensor is attached to the vicinity of a workpiece to be forged. The workpiece is typically processed at a high temperature of about 1,200° C. to 1,000° C. The sensor provided near the workpiece is affected by scattering of scales or heat so that the sensor cannot detect the opening degree of the holding claws stably.
In addition, one end of the operating rod to which the detection plate is attached moves vertically while the other end thereof moves circularly. Therefore, the motion of the detection plate is complicated during the opening/closing motion of the holding claws. Thus, the range where the position of the operating rod can be detected is limited to a narrow range.
Patent Document 1: Japanese Patent No. 5544946
Patent Document 2: JP-A-2013-43228
Patent Document 3: JP-A-2000-94070
Patent Document 4: JP-A-2013-78791